To be honest, it's all about experimentation. I'm no Photoshop buff, I just added a few filters here and there, see what worked, tweaked the brightness/contrast and voila!
Here's a rough explanation of what I did:
- First of all, Image > Adjustments > Desaturate;
- Increased the brightness/contrast;
- Duplicated the layer, added grain (vertical) - you need to experiment with the grain and see what works best;
- Set the layer to overlay and merged with bottom layer - again, try different blending modes before merging;
- Created a new transparent layer;
- Rendered clouds, followed by difference clouds;
- Set the transparency of the layer to +-50%, merged with bottom layer;
- Created new layer, Filter > Render > Fibres. Experiment with Variance and Strength;
- Set the blending mode to Soft Light, transparency of layer +- 50%, merged with bottom layer;
- You can add some Film Grain if you want to;
- Add some spots, black or white to the image with the brush for the final touch (grunge brushes works the best, you'll have to google for them though);
The result:
